Fermi telescope measures cosmic 'fog' produced by ancient starlight

Nov 01, 2012
This plot shows the locations of 150 blazars (green dots) used in the EBL study. The background map shows the entire sky and was constructed from four years of gamma rays with energies above 10 billion electron volts detected by Fermi. The plane of our Milky Way galaxy runs along the middle of the plot. The Fermi LAT instrument is the first to detect more than 500 sources in this energy range. Credit: NASA/DOE/Fermi LAT Collaboration

(Phys.org)—Astronomers using data from NASA's Fermi Gamma-ray Space Telescope have made the most accurate measurement of starlight in the universe and used it to establish the total amount of light from all of the stars that have ever shone, accomplishing a primary mission goal.

"The optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine, and this creates a fossil radiation field we can explore using from distant sources," said lead scientist Marco Ajello, a postdoctoral researcher at the Kavli Institute for and Cosmology at Stanford University in California and the Space Sciences Laboratory at the University of California at Berkeley.

Gamma rays are the most energetic form of light. Since Fermi's launch in 2008, its Large Area Telescope (LAT) observes the entire sky in high-energy gamma rays every three hours, creating the most detailed ever known at these energies.

The total sum of starlight in the cosmos is known to astronomers as the extragalactic background light (EBL). To gamma rays, the EBL functions as a kind of cosmic fog. Ajello and his team investigated the EBL by studying gamma rays from 150 blazars, or galaxies powered by , that were strongly detected at energies greater than 3 billion (GeV), or more than a billion times the energy of visible light.

"With more than a thousand detected so far, blazars are the most common sources detected by Fermi, but gamma rays at these energies are few and far between, which is why it took four years of data to make this analysis," said team member Justin Finke, an astrophysicist at the Naval Research Laboratory in Washington.

This video is not supported by your browser at this time.
This animation tracks several gamma rays through space and time, from their emission in the jet of a distant blazar to their arrival in Fermi's Large Area Telescope (LAT). During their journey, the number of randomly moving ultraviolet and optical photons (blue) increases as more and more stars are born in the universe. Eventually, one of the gamma rays encounters a photon of starlight and the gamma ray transforms into an electron and a positron. The remaining gamma-ray photons arrive at Fermi, interact with tungsten plates in the LAT, and produce the electrons and positrons whose paths through the detector allows astronomers to backtrack the gamma rays to their source. Credit: NASA's Goddard Space Flight Center/Cruz deWilde

As matter falls toward a galaxy's , some of it is accelerated outward at almost the speed of light in jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, the galaxy appears especially bright and is classified as a blazar.

Gamma rays produced in blazar jets travel across billions of light-years to Earth. During their journey, the gamma rays pass through an increasing fog of visible and ultraviolet light emitted by stars that formed throughout the history of the universe.

Occasionally, a gamma ray collides with starlight and transforms into a pair of particles—an electron and its antimatter counterpart, a positron. Once this occurs, the gamma ray light is lost. In effect, the process dampens the gamma ray signal in much the same way as fog dims a distant lighthouse.

From studies of nearby blazars, scientists have determined how many gamma rays should be emitted at different energies. More distant blazars show fewer gamma rays at higher energies—especially above 25 GeV—thanks to absorption by the cosmic fog.

The farthest are missing most of their higher-.

The researchers then determined the average gamma-ray attenuation across three distance ranges between 9.6 billion years ago and today.

Fermi measured the amount of gamma-ray absorption in blazar spectra produced by ultraviolet and visible starlight at three different epochs in the history of the universe. Credit: NASA's Goddard Space Flight Center

From this measurement, the scientists were able to estimate the fog's thickness. To account for the observations, the average stellar density in the cosmos is about 1.4 stars per 100 billion cubic light-years, which means the average distance between stars in the universe is about 4,150 light-years.

A paper describing the findings was published Thursday on Science Express.

"The Fermi result opens up the exciting possibility of constraining the earliest period of cosmic star formation, thus setting the stage for NASA's James Webb Space Telescope," said Volker Bromm, an astronomer at the University of Texas, Austin, who commented on the findings. "In simple terms, Fermi is providing us with a shadow image of the first stars, whereas Webb will directly detect them."

Measuring the extragalactic background was one of the primary mission goals for Fermi.

"We're very excited about the prospect of extending this measurement even farther," said Julie McEnery, the mission's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

Explore further: Raven soars through first light and second run

More information: "The Imprint of The Extragalactic Background Light in the Gamma-Ray Spectra of Blazars," by M. Ackermann, www.sciencemag.org/content/ear… 0/31/science.1227160

Related Stories

Fermi telescope explores new energy extremes

Jan 10, 2012

(PhysOrg.com) -- After more than three years in space, NASA's Fermi Gamma-ray Space Telescope is extending its view of the high-energy sky into a largely unexplored electromagnetic range. Today, the Fermi ...

600 mysteries in the night sky

Oct 19, 2011

NASA's Fermi team recently released the second catalog of gamma-ray sources detected by their satellite's Large Area Telescope (LAT). Of the 1873 sources found, nearly 600 are complete mysteries. No one knows ...

Fermi Telescope probes 'dragons' of the gamma-ray sky

Mar 02, 2010

One of the pleasures of perusing ancient maps is locating regions so poorly explored that mapmakers warned of dragons and sea monsters. Now, astronomers using NASA's Fermi Gamma-ray Space Telescope find themselves ...

Recommended for you

Raven soars through first light and second run

3 hours ago

Raven, a Multi-Object Adaptive Optics (MOAO) science demonstrator, successfully saw first light at the Subaru Telescope on the nights of May 13 and 14, 2014 and completed its second run during the nights ...

How can we find tiny particles in exoplanet atmospheres?

Aug 29, 2014

It may seem like magic, but astronomers have worked out a scheme that will allow them to detect and measure particles ten times smaller than the width of a human hair, even at many light-years distance.  ...

User comments : 14

Adjust slider to filter visible comments by rank

Display comments: newest first

Pressure2
2.3 / 5 (6) Nov 01, 2012
This article gives the best definition of a blazar I have ever read anywhere. A very interesting article.

Quote from article: "As matter falls toward a galaxy's supermassive black hole, some of it is accelerated outward at almost the speed of light in jets pointed in opposite directions. When one of the jets happens to be aimed in the direction of Earth, the galaxy appears especially bright and is classified as a blazar."
Silverhill
2 / 5 (8) Nov 01, 2012
Quote from article:
In effect, the process dampens the gamma ray signal in much the same way as fog dims a distant lighthouse.
No, it does not. The gamma rays do not become damp (or moist, or wet).
They do become *damped*, however. "Damp: to check or retard the energy, action, etc., of"

Q-Star
1 / 5 (5) Nov 01, 2012
This article gives the best definition of a blazar I have ever read anywhere. A very interesting article.


I agree, it's one of those "I should have been able to say that" sort of things. You never see it until "someone else" says it first.
Q-Star
2.8 / 5 (9) Nov 01, 2012
No, it does not. The gamma rays do not become damp (or moist, or wet).
They do become *damped*, however. "Damp: to check or retard the energy, action, etc., of"


I'm pretty sure that using the word "dampen" to mean attenuate, cause to be less vigorous, or to lessen is correct. At least the OED says so.
Silverhill
1 / 5 (4) Nov 01, 2012
"Dampen" is listed as a synonym of "damp", but it should not be(IMHO). In a chimney, for instance, there may be a "damper", but not a "dampener". A gardener might "dampen" dry soil, but would not "damp" it.
There is no need to conflate these verbs.
Sonhouse
5 / 5 (1) Nov 01, 2012
People have to find SOME flaw in an article to get their name in the lights for a few minutes. Silverhill just got his 15 minutes of flame.
Silverhill
1 / 5 (3) Nov 01, 2012
It's not about fame (or flame), Sonhouse; I don't care whether my handle appears here. Since the moderators don't seem to respond to comments directed straight to them, I put things (including gripes about various solecisms) here in hopes that they might become better aware of the substandard "product" they're offering.
lengould100
5 / 5 (1) Nov 02, 2012
Weird. Some complain about the author`s science education, other(s) about their language skills.

The readers here should make up their minds. Finding an author who can describe blazers while avoiding solecisms willing to write for us is bound to be extremely difficult.
Sonhouse
5 / 5 (3) Nov 02, 2012
Silverhill:
"dampens:3rd person singular present of damp·en (Verb)
Verb:
Make slightly wet: "the fine rain dampened her face".
Make less strong or intense: "nothing could dampen her enthusiasm"." From the Webster online dictionary.

Please tell me again what is wrong with using 'Dampen' in the sense they were saying in the article.

That is not even close to being a solecism.
cantdrive85
1 / 5 (10) Nov 02, 2012
The most amazing thing about gravity, is not only can it create a force of attraction so large that not even light can escape, but it (at the same time) can create a repulsive force even stronger than the attractive force to form collimated jets that sends radiation at relativistic speeds in two directions. It's just a remarkable display of a contradictory force. Maybe EM is just an extension of the all powerful gravitational force, as a matter of fact, I bet the strong force is just the atomic representation of spooky gravity. Gravity is soooo amazing in all that it can do, it's like the anti-EM of the electrical theorist. It can repel and attract all at the same time, just like EM can.
wavettore
1 / 5 (7) Nov 02, 2012
New and old Science

A new and Progressive Science shows how Wavevolution, or the transformation from waves to atoms, is the connecting link that closes the circle of science to open our eyes toward new horizons never seen before.

The bureaucracy of traditional science prevents the recognition of any event unless certain criteria are first met. The problem of this science is buried deep in the compilation of these "laws" or criteria introduced by a few scientists in the name of all science and from their erroneous understanding of the relation between Space and Time. This antiquated system of rules also results in misleading theories.
barakn
5 / 5 (3) Nov 02, 2012
Cantdrive85 is incapable of admitting when astrophysics use electromagnetism to explain things, because that would bust the EU myth that they don't. There's nothing worse to an EU acolyte than to have the founding myths busted. So the idiot is going to sit in front of his computer and pretend that the black hole model doesn't involve the black hole's gravity pulling in, concentrating, and twisting magnetic fields to the point that charged particles can utilize the stored energy of those fields to blast away at almost light speed.
Q-Star
1 / 5 (4) Nov 02, 2012
A new and Progressive Science shows how Wavevolution, or the transformation from waves to atoms, is the connecting link that closes the circle of science to open our eyes toward new horizons never seen before.


From waves to atoms? You should have said "blah, blah, blah and blah"? Would have made more sense.

The bureaucracy of traditional science prevents the recognition of any event unless certain criteria are first met.


The criteria is an event that is reproducible and subject to an explanation that can be falsified, experimentally tested. Other wise it is just metaphysical musings. (Most often its just sophomoric metaphysical musing.)

The problem of this science is buried deep in the compilation of these "laws" or criteria introduced by a few scientists in the name of all science and from their erroneous understanding of the relation between Space and Time. This antiquated system of rules also results in misleading theories.


Back to "blah, blah, & blah"
Silverhill
1 / 5 (2) Nov 02, 2012
Sonhouse
Dampen: Make less strong or intense: "nothing could dampen her enthusiasm"
In this sense, it could be used. Dampen, in this example, is a figurative use of the effect of putting water on a fire, or putting a real (or metaphoric) "wet blanket" on someone/something.
My point remains: "damp" means *only* "Make less strong or intense", and is already an appropriate verb for describing the diminution of energy of the traveling gamma photons. There is little need to co-opt a related, but less precise, verb.